Circuit and method for operating a delay-lock loop in a power saving manner

ABSTRACT

A control circuit for a delay-lock loop having a delay line and a phase detector is used in a memory device. In a standby mode, the control circuit isolates a reference clock signal from the delay-lock loop to save power unless a clock signal generated by the loop is needed for a memory operation. However, the reference signal is periodically coupled to the delay line for a sufficient period to achieve a locked condition. As a result, the phase of the output signal from delay-lock loop can be quickly locked to the phase of the reference signal when a memory operation is to occur during a normal operating mode. When transitioning between the standby mode and the normal operating mode, the control circuit couples the reference clock signal to the delay line for at least a predetermined period of time.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of pending U.S. patent application Ser.No. 11/077,641, filed Mar. 11, 2005.

TECHNICAL FIELD

The present invention is directed to memory and other electronic devicesemploying locked loops such as delay-lock loops (“DLL”), and moreparticularly, to a circuit and method for operating such loops in amanner that minimizes power consumption in such devices.

BACKGROUND OF THE INVENTION

Periodic digital signals are commonly used in a variety of electronicdevices, such as memory devices. Probably the most common of periodicdigital signals are clock signals that are typically used to establishthe timing of a digital signal or the timing at which an operation isperformed on a digital signal. For example, data signals are typicallycoupled to and from memory devices, such as synchronous dynamic randomaccess memory (“SDRAM”) devices, in synchronism with a clock or datastrobe signal.

As the speed of memory devices and other devices continue to increase,the “eye” or period in which a digital signal, such as a data signal, isvalid becomes smaller and smaller, thus making the timing of a strobesignal or other clock signal used to capture the digital signal evenmore critical. In particular, as the size of the eye becomes smaller,the propagation delay of the strobe signal can be different from thepropagation delay of the captured digital signal(s). As a result, theskew of the strobe signal relative to the digital signal can increase tothe point where a transition of the strobe signal is no longer withinthe eye of the captured signal.

One technique that has been used to ensure the correct timing of astrobe signal relative to captured digital signals is to use a lockedloop, such as a delay-lock loop (“DLL”), to generate the strobe signalin particular, a delay-lock loop allows the timing of the strobe signalto be adjusted to minimize the phase error between the strobe signal andthe valid eye of the digital signal. A typical delay-lock loop uses adelay line (not shown) consisting of a large number of delay stages. Areference clock signal is applied to the delay line, and it propagatesthrough the delay line to the final delay stage, which outputs a delayedclock signal. The phase of the delayed clock signal is compared to thephase of the reference clock signal to generate a phase error signal.The phase error signal is used to adjust the delay provided by the delaystages in the delay line until the phase of the delayed clock signal isequal to the phase of the reference clock signal.

Another problem associated with the high operating speed of memory andother devices is excessive power consumption, particularly for portableelectronic devices like notebook or other portable computers. Asignificant amount of power consumption in portable computers is theresult of power consumed by DRAM devices, which are normally used assystem memory. It is therefore important to minimize the power consumedby DRAM devices so that such computers can be powered by batteries overan extended period. Excessive power consumption can also create problemseven where DRAM devices are not powered by batteries. For example, theheat generated by excessive power consumption can damage the DRAMdevices, and it can be difficult and/or expensive to maintain thetemperature of electronic equipment containing the DRAM devices at anacceptably low value.

Power is consumed each time a digital circuit is switched to change thelogic level of a digital signal. The rate at which power is consumed byDRAM and other memory devices therefore increases with both theoperating speed of such devices and the number of circuits beingswitched. Thus, the demands for ever increasing operating speeds andmemory capacity are inconsistent with the demands for ever decreasingmemory power consumption.

Various circuits in DRAM devices consume power at various rates. Asignificant amount of power is consumed by locked loops, particularlydelay-lock loops, which, as explained above are commonly used in DRAMdevices. Delay-lock loops consume a great deal of power because thedelay lines used in such loops often contain a large number of delaystages, all of which are switched as a reference clock signal propagatesthrough the delay line. The higher reference clock signal frequenciesneed to operate the DRAM devices at higher speed causes these largenumber delay stages to be switched at a rapid rate, thereby consumingpower at a rapid rate.

A number of techniques have been used to reduce power consumption inDRAM devices while allowing for increases in operating speeds and memorycapacity. One approach has been to prevent digital circuits fromswitching when such circuits are not active since, as mentioned above,power is consumed each time a component in the digital circuit isswitched from one state to another. While this approach cansignificantly reduce the power consumed by DRAM devices, there arecircuits in DRAM devices that cannot be rendered inactive withoutcompromising the speed and/or operability of the DRAM devices.Delay-lock loops, for example, often cannot be switched off because ofthe amount of time needed for the loops to achieve a locked conditionafter they have been powered down for a considerable period. For thesereasons, the coupling of a reference clock signals to delay-lock loopshave traditionally been terminated to reduce power consumption only whenthere is some assurance that it will not be necessary for the DRAMdevice to read or write data for a considerable period. For example,DRAM devices have been placed in a power-down state when the DRAM deviceswitches to a self-refresh mode or when a computer system containing theDRAM devices switches to a low power standby mode. However, there areother times where the clock signals produced by delay-lock loops are notactually needed, and additional power savings could be achieved.Furthermore, removing the reference clock signals from delay-lock loopsfor long periods even during extended periods like self-refresh allowsthe delay of the delay lines used in the delay-lock loops to changeconsiderably, thus requiring an undesirably long period for thedelay-lock loop to again achieve a locked condition.

There is therefore a need for a method and system for allowing areference clock signal to be removed from delay-lock loops to a greaterextent, thereby further reducing power consumption, without sacrificingoperating speed or performance resulting from the time needed for theloop to achieve a locked condition.

SUMMARY OF THE INVENTION

A circuit and method of operating a delay-lock loop includes a memorydevice in either a normal mode or a standby mode. In the normal mode, areference clock signal is continuously coupled to a delay line used inthe delay-lock loop. In the standby mode, the reference clock signal isgenerally isolated from the delay line so that the delay line does notconsume power switching state responsive to the reference clock signal.However, the reference clock signal is periodically coupled to the delayline in the standby mode for an update period of sufficient duration toallow the delay-lock loop to achieve a locked condition. When enteringthe normal operating mode, the reference clock signal is coupled to thedelay line for at least a predetermined period having a sufficientduration for the delay-lock loop to achieve a locked condition beforethe reference clock signal can again be isolated from the delay line.The normal operating mode is entered responsive to detecting a memoryoperation requiring a clock signal generated by the delay-lock loop,such as when a bank of memory cells in the memory device becomes active.When entering the standby mode, the reference signal is immediatelyisolated from the delay line if the loop is already locked. Otherwise,the reference signal remains coupled to the delay line for a sufficientperiod for the loop to become locked prior to being isolated from thedelay line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a control circuit for operating adelay-lock loop in a power saving mode according to one example of theinvention.

FIG. 2 is a block diagram of a memory device using the delay-lock loopand control circuit of FIG. 1 or some other example of the invention.

FIG. 3 is a block diagram of a computer system using the memory deviceof FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

A system 10 of controlling the operation of a delay-lock loop tominimize power consumption according to one example of the invention isshown in FIG. 1. The system 10 includes a delay-lock loop 12 having avoltage controlled delay line 14, which delays a CLK-IN signal appliedto its input by a delay time determined by a control signal applied toits control “C” input. The control signal is supplied by a delay controlcircuit 16 based on an error signal. The error signal is generated by aphase detector 18, and it has a value corresponding to the differencebetween the phase of a CLK signal applied to one of its inputs and thephase of a CLK-OUT signal applied to the other of its inputs.

In operation, whenever the phase detector 18 is enabled by an activehigh signal applied to its E input, the phase detector 18 and delaycontrol circuit 16 set the delay of the voltage controlled delay line 14so that the phase of the CLK signal is equal to the phase of the CLK-OUTsignal.

The operation of the delay-lock loop 12 is controlled by a loop controlcircuit 20. The loop control circuit 20 selectively enables the phasedetector 18 with a phase detector On (“PDOn”) signal generated by alogic circuit 22 and coupled through an inverter 23 and NAND gate 24.The logic circuit 22 receives several control signals CONT, the natureof which will be described in greater detail below. The logic circuit 22also outputs a delay-lock loop On (“DLLOn”) signal, which is applied toone input of a NAND gate 26. The NAND gate 26 also receives a Mode Ensignal and a reference clock RefCLK signal. The Mode En signal is alsoapplied through an inverter 28 to one input of a NAND gate 30, whichalso receives the RefCLK signal, and to the other input of the NAND gate24. The RefCLK signal coupled to the output of either of the NAND gates26, 30 is applied to a NAND gate 32, which then outputs the CLK-INsignal to the DLL 12.

The Mode En signal is generated by a mode register (not shown in FIG. 1)that is typically used in DRAM devices. The mode register is programmedby a user to enable certain functions, including in this case, theability to selectively power down the DLL 12. The Mode En signal enableseither the NAND gates 24, 26 or the NAND gate 30, but not all three atthe same time. When the Mode En signal is inactive low, the NAND gates24, 26 are disabled so that they each output a high. The high at theoutput of the NAND gate 24 continuously enables the phase detector 18regardless of the state of the PDOn signal. The high at the output ofthe NAND gate 26 enables the NAND gate 32. The low Mode En signal alsoenables the NAND gate 30 through the inverter 28 so that the RefCLKsignal is coupled to the output of the enabled NAND gate 32. Thus,whenever, the Mode En signal is inactive low, the phase detector 18 iscontinuously enabled, and the RefClk signal is coupled to the voltagecontrolled delay line 14 regardless of the state of the DLLOn and PDOnsignals from the logic circuit 22.

When the Mode En signal is active high, the NAND gate 30 is disabledthrough the inverter 28, thereby outputting a high to enable the NANDgate 32. The high Mode En signal also enables the NAND gates 24, 26.Under these circumstances the NAND gate 24 outputs a high to enable thephase detector 18 whenever the PDOn signal is active high.Alternatively, the NAND gate 24 outputs a low to disable the phasedetector 18 whenever the PDOn signal is inactive low. In enabling theNAND gate 26, the high Mode En signal causes the NAND gate 26 to couplethe RefCLK signal through the NAND gate 32 to generate the CLK-IN signalwhenever the DLLOn signal is active high. Whenever the DLLOn signal isinactive low, the NAND gate 26 is disabled to terminate the CLK-INsignal.

In summary, when the Mode En signal is inactive low, the DLL 12 iscontinuously enabled. When the Mode En signal is active high, the DLLOnsignal selectively causes the CLK-IN signal to be coupled to the voltagecontrolled delay line 14, and the PDOn signal selectively enables thephase detector 18. The DLLOn and PDOn signals are selectively switchedto active and inactive states based upon a number of control signalsCONT, which are indicative of the operation of a DRAM in which thesystem 10 is included.

The nature of the CONT signals that cause the logic to make the DLLOnand PDOn signals active or inactive will now be described with referenceto the operating state of the DRAM in which the system 10 is included.It will be understood that these operating states are implemented by thecontrol signals CONT that are applied to the logic circuit 22 in theloop control circuit 20. The DLLOn and PDOn signals are generated tocouple the RefCLK signal to the delay line 14 and enable the phasedetector 18, respectively, in a normal operating mode and in a standbymode as follows:

-   -   In the normal operating mode, the DLLOn and PDOn signals are        continuously generated to couple the RefCLK signal to the delay        line 14 and enable the phase detector 18.    -   When entering the normal operating mode, the DLLOn and PDOn        signals are generated for at least a predetermined minimum        period even if the standby mode is entered shortly thereafter.        In one embodiment, this minimum period is 256 periods of a        system clock signal. This prevents the DLL 12 from being turned        on and off rapidly, which might allow the DLL 12 to operate in a        spurious manner. The only exception is if the DLL 12 is already        locked. If the DLL is locked, then the DLLOn and PDOn signals        can immediately terminate as soon as the standby mode is        entered.    -   In the standby mode, the DLLOn and PDOn signals are terminated        to isolate the RefCLK signal from the delay line 14 and disable        the phase detector 18 for a predetermined power-down period,        which may be about 4,000 cycles of a system clock. After the        power-down period, the DLLOn and PDOn signals are generated to        coupled the RefCLK signal to the delay line 14 and enable the        phase detector 18 for an update period of sufficient duration to        allow the DLL 12 to achieve a locked condition. In one example        of the DLL 12, the duration of the update period is 256 cycles        of the system clock.    -   The normal operating mode is entered if a bank of memory cells        becomes active.    -   The normal operating mode is also entered for a relatively long        update period if the DLL 12 is reset, which ensures that the DLL        12 can achieve a locked condition. In one embodiment, the        duration of the long update period is 1,000 cycles of the system        clock. This long update period ensures that the DLL 12 has        sufficient time to find a good lock point.    -   When exiting a power down period or when exiting a self-refresh        period, the DLLOn and PDOn signals are generated for the long        refresh period to couple the RefCLK signal to the delay line 14        and enable the phase detector 18 for the long update period.    -   Whenever an on die termination (“ODT”) feature is enabled for a        DRAM containing the DLL 12, the CLK-OUT signal is needed.        However, the phase of the CLK-OUT signal need only be        approximately correct. For this reason, the DLLOn signal is        generated so that the RefCLK signal propagates through the delay        line 14 to produce the CLK-OUT signal. The PDOn signal is not        generated so the phase detector 18 remains disabled since there        is no need to precisely adjust the phase of the CLK-OUT signal.    -   The DLLOn and PDOn signals may be generated whenever a load mode        (“LDMD”) command is applied to the DRAM containing the DLL 12        since the DLL 12 is reset by setting a bit in the mode register        of the DRAM's command decoder.

Delay-lock loops according to various embodiments of the presentinvention can be used for a variety of purposes in electronic devices,such as memory devices. For example, with reference to FIG. 2, asynchronous dynamic random access memory (“SDRAM”) 100 includes acommand decoder 104 that controls the operation of the SDRAM 100responsive to high-level command signals received on a control bus 106and coupled through input receivers 108. These high level commandsignals, which are typically generated by a memory controller (not shownin FIG. 2), are a clock enable signal CKE*, a clock signal CLK, a chipselect signal CS*, a write enable signal WE*, a row address strobesignal RAS*, a column address strobe signal CAS*, and a data mask signalDQM, in which the “*” designates the signal as active low. The commanddecoder 104 generates a sequence of command signals responsive to thehigh level command signals to carry out the function (e.g., a read or awrite) designated by each of the high level command signals. Thesecommand signals, and the manner in which they accomplish theirrespective functions, are conventional. Therefore, in the interest ofbrevity, a further explanation of these command signals will be omitted.

The command decoder 104 also includes a mode register 105 that can beprogrammed by a user to control the operating modes and operatingfeatures of the SDRAM 100. The mode register 105 is programmedresponsive to a load mode (“LDMD”) command, which is registeredresponsive to a predetermined combination of the command signals appliedto the command decoder 104 through the control bus 106. One of theoperating features that can be programmed into the mode register is thepreviously described on die termination (“ODT”) feature. As alsopreviously described, the mode register 105 is programmed by setting apredetermined bit responsive to the load mode command to reset the DLL12. It is for that reason the DLLOn and PDOn signals are generatedwhenever a load mode command is decoded, as described above.

The SDRAM 100 includes an address register 112 that receives rowaddresses and column addresses through an address bus 114. The addressbus 114 is generally coupled through input receivers 110 and thenapplied to a memory controller (not shown in FIG. 2). A row address isgenerally first received by the address register 112 and applied to arow address multiplexer 118. The row address multiplexer 118 couples therow address to a number of components associated with either of twomemory banks 120, 122 depending upon the state of a bank address bitforming part of the row address. Associated with each of the memorybanks 120, 122 is a respective row address latch 126, which stores therow address, and a row decoder 128, which decodes the row address andapplies corresponding signals to one of the arrays 120 or 122. The rowaddress multiplexer 118 also couples row addresses to the row addresslatches 126 for the purpose of refreshing the memory cells in the arrays120, 122. The row addresses are generated for refresh purposes by arefresh counter 130, which is controlled by a refresh controller 132.The refresh controller 132 is, in turn, controlled by the commanddecoder 104.

After the row address has been applied to the address register 112 andstored in one of the row address latches 126, a column address isapplied to the address register 112. The address register 112 couplesthe column address to a column address latch 140. Depending on theoperating mode of the SDRAM 100, the column address is either coupledthrough a burst counter 142 to a column address buffer 144, or to theburst counter 142 which applies a sequence of column addresses to thecolumn address buffer 144 starting at the column address output by theaddress register 112. In either case, the column address buffer 144applies a column address to a column decoder 148.

Data to be read from one of the arrays 120, 122 is coupled to the columncircuitry 154, 155 for one of the arrays 120, 122, respectively. Thedata is then coupled through a data output register 156 and data outputdrivers 157 to a data bus 158. The data output drivers 157 apply theread data to the data bus 158 responsive to a read data strobe signalS_(R) generated by the delay-lock loop 12 included in the delay-lockloop control system 10 or some other example of the invention. The SDRAM100 shown in FIG. 2 is a double data rate (“DDR”) SDRAM that inputs oroutputs data twice each clock period. The delay-lock loop control system10 receives the periodic RefCLK signal and generates the read datastrobe S_(R) with a phase that is substantially equal to the phase ofthe RefCLK signal. As a result, the read data are coupled to the databus 158 substantially in phase with the RefCLK signal.

Data to be written to one of the arrays 120, 122 are coupled from thedata bus 158 through data input receivers 161 to a data input register160. The data input receivers 161 couple the write data from the databus 158 responsive to a write data strobe signal S_(W) generated by asecond delay-lock loop 12 in the delay-lock loop control system 10 or bysome other example of the invention. The delay-lock loop 12 in thecontrol system 10 receives the periodic RefCLK signal and generates thewrite data strobe S_(W) with a phase that is substantially thequadrature of the phase of the RefCLK signal. As a result, the writedata are coupled into the SDRAM 100 from the data bus 158 at the centerof a “data eye” corresponding to the phase of the RefCLK signal. Thewrite data are coupled to the column circuitry 154, 155 where they aretransferred to one of the arrays 120, 122, respectively. A mask register164 responds to a data mask DM signal to selectively alter the flow ofdata into and out of the column circuitry 154, 155, such as byselectively masking data to be read from the arrays 120, 122.

The SDRAM 100 shown in FIG. 2 can be used in various electronic systems.For example, it may be used in a processor-based system, such as acomputer system 200 shown in FIG. 3. The computer system 200 includes aprocessor 202 for performing various computing functions, such asexecuting specific software to perform specific calculations or tasks.The processor 202 includes a processor bus 204 that normally includes anaddress bus, a control bus, and a data bus. In addition, the computersystem 200 includes one or more input devices 214, such as a keyboard ora mouse, coupled to the processor 202 to allow an operator to interfacewith the computer system 200. Typically, the computer system 200 alsoincludes one or more output devices 216 coupled to the processor 202,such output devices typically being a printer or a video terminal. Oneor more data storage devices 218 are also typically coupled to theprocessor 202 to allow the processor 202 to store data in or retrievedata from internal or external storage media (not shown). Examples oftypical storage devices 218 include hard and floppy disks, tapecassettes, and compact disk read-only memories (CD-ROMs). The processor202 is also typically coupled to cache memory 226, which is usuallystatic random access memory (“SRAM”), and to the SDRAM 100 through amemory controller 230. The memory controller 230 normally includes acontrol bus 236 and an address bus 238 that are coupled to the SDRAM100. A data bus 240 is coupled from the SDRAM 100 to the processor bus204 either directly (as shown), through the memory controller 230, or bysome other means.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, it will be understood by one skilled in the art thatvarious modifications may be made without deviating from the spirit andscope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

1. A method of operating a delay-lock loop included in a memory device,the method comprising: in a normal operating mode of the memory device,continuously coupling a reference clock signal to a delay line used inthe delay-lock loop; and in a standby mode of the memory device:entering a standby period in which the reference clock signal isisolated from the delay line; and while in the standby period,periodically coupling the reference clock signal to the delay line foran update period of sufficient duration to allow the delay-lock loop toachieve a locked condition, and then re-entering the standby period. 2.The method of claim 1, further comprising, when the memory device entersthe normal operating mode, coupling the reference clock signal to thedelay line for at least a predetermined period.
 3. The method of claim1, further comprising: detecting when a bank of memory cells in thememory device becomes active when the memory device is in the standbymode; and coupling the reference clock signal to the delay line used inthe delay-lock loop responsive to detecting that the bank of memorycells has become active.
 4. The method of claim 1, further comprising:determining if the delay-lock loop is in a locked condition whenentering the standby mode; if the determination is made that thedelay-lock loop is in a locked condition when entering the standby mode,immediately isolating the reference clock signal from the delay line;and if the determination is made that the delay-lock loop is not in alocked condition when entering the standby mode, continuing to couplethe reference clock signal to the delay line for a predetermined periodbefore isolating the reference clock signal from the delay line.
 5. Themethod of claim 1, further comprising: determining if the delay-lockloop is being reset when the memory device is in the standby mode; andin response to determining that the delay-lock loop is being reset,coupling the reference clock signal to the delay line for a reset periodbefore isolating the reference clock signal from the delay line.
 6. Themethod of claim 5 wherein the reset period is substantially longer thanthe update period.
 7. The method of claim 1, further comprising:determining if memory device is transitioning from the standby mode tothe normal operating mode; and in response to determining that thememory device is transitioning from the standby mode to the normaloperating mode, coupling the reference clock signal to the delay linefor a period that is substantially longer than the update period.
 8. Themethod of claim 1 wherein the memory device is operable in a refreshmode in which refresh signals are generated internally in the memorydevice, and wherein the method further comprises: determining if memorydevice is transitioning from the refresh mode in which refresh signalsare generated internally in the memory device, and in response todetermining that the memory device is transitioning out of the refreshmode in which refresh signals are generated internally in the memorydevice, coupling the reference clock signal to the delay line for aperiod that is substantially longer than the update period.
 9. Themethod of claim 8 wherein the refresh mode in which refresh signals aregenerated internally in the memory device comprises a self-refresh mode.10. The method of claim 1 wherein the delay-lock loop further comprise aphase detector having first and second input terminals receiving thereference clock signal and an output signal from the delay line, thephase detector being operable to generate an output signal that controlsthe delay of the delay line, the method further comprising selectivelyenabling and disabling the phase detector when the memory device isoperating in the standby mode.
 11. The method of claim 10 wherein thememory device is operable in an on die termination mode, and wherein themethod further comprises disabling the phase detector in the on dietermination mode while coupling the reference clock signal to the delayline when the memory device is operating in the standby mode.
 12. Themethod of claim 1 wherein the memory device comprises a mode registerthat is programmable responsive to a load mode command, and wherein themethod further comprises: detecting the load mode command, and when inthe standby mode, coupling the reference clock signal to the delay lineused in the delay-lock loop responsive to detecting the load modecommand.
 13. A method of operating a delay-lock loop included in amemory device, the method comprising: in a standby mode of the memorydevice, isolating the reference clock signal from a delay line used inthe delay-lock loop; and in a normal operating mode of the memorydevice, continuously coupling the reference clock signal to the delayline for at least a predetermined period regardless of whether or notthe memory device remains in the normal operating mode.
 14. The methodof claim 13, further comprising: detecting when a bank of memory cellsin the memory device becomes active when the memory device is in thestandby mode; coupling the reference clock signal to the delay line usedin the delay-lock loop responsive to detecting that the bank of memorycells has become active.
 15. The method of claim 13, further comprising:determining if the delay-lock loop is in a locked condition whenentering the standby mode; if the determination is made that thedelay-lock loop is in a locked condition when entering the standby mode,immediately isolating the reference clock signal from the delay line;and if the determination is made that the delay-lock loop is not in alocked condition when entering the standby mode, continuing to couplethe reference clock signal to the delay line for a predetermined periodbefore isolating the reference clock signal from the delay line.
 16. Themethod of claim 13, further comprising: determining if the delay-lockloop is being reset when the memory device is in the standby mode; andin response to determining that the delay-lock loop is being reset,coupling the reference clock signal to the delay line for a reset periodbefore isolating the reference clock signal from the delay line, thereset period being longer than the predetermined period during which thereference clock signal is coupled to the delay line.
 17. The method ofclaim 13, further comprising: determining if memory device istransitioning from the standby mode to the normal operating mode; and inresponse to determining that the memory device is transitioning from thestandby mode to the normal operating mode, coupling the reference clocksignal to the delay line for a period that is substantially longer thanthe predetermined period during which the reference clock signal iscoupled to the delay line.
 18. The method of claim 13 wherein the memorydevice is operable in a refresh mode in which refresh signals aregenerated internally in the memory device, and wherein the methodfurther comprises: determining if memory device is transitioning fromthe refresh mode in which refresh signals are generated internally inthe memory device, and in response to determining that the memory deviceis transitioning out of the refresh mode in which refresh signals aregenerated internally in the memory device, coupling the reference clocksignal to the delay line for a period that is substantially longer thanthe predetermined period during which the reference clock signal iscoupled to the delay line.
 19. The method of claim 18 wherein therefresh mode in which refresh signals are generated internally in thememory device comprises a self-refresh mode.
 20. The method of claim 13wherein the delay-lock loop further comprise a phase detector havingfirst and second input terminals receiving the reference clock signaland an output signal from the delay line, the phase detector beingoperable to generate an output signal that controls the delay of thedelay line, the method further comprising selectively enabling anddisabling the phase detector when the memory device is operating in thestandby mode.
 21. The method of claim 20 wherein the memory device isoperable in an on die termination mode, and wherein the method furthercomprises disabling the phase detector in the on die termination modewhile coupling the reference clock signal to the delay line when thememory device is operating in the standby mode.
 22. The method of claim13 wherein the memory device comprises a mode register that isprogrammable responsive to a load mode command, and wherein the methodfurther comprises: detecting the load mode command, and when in thestandby mode, coupling the reference clock signal to the delay line usedin the delay-lock loop for at least the predetermined period responsiveto detecting the load mode command.
 23. In a memory device having adelay-lock loop including a delay line, a method of transitioning thememory device from a normal operating mode in which a reference clocksignal is coupled to the delay line to a standby mode in which thereference clock signal is isolated from the delay line, the methodcomprising: determining if the delay-lock loop is in a locked conditionwhen entering the standby mode; if the determination is made that thedelay-lock loop is in a locked condition when entering the standby mode,immediately isolating the reference clock signal from the delay line;and if the determination is made that the delay-lock loop is not in alocked condition when entering the standby mode, continuing to couplethe reference clock signal to the delay line for a predetermined periodbefore isolating the reference clock signal from the delay line.
 24. Themethod of claim 23, further comprising: detecting when a bank of memorycells in the memory device becomes active when the memory device is inthe standby mode; and coupling the reference clock signal to the delayline used in the delay-lock loop responsive to detecting that the bankof memory cells has become active.
 25. The method of claim 23, furthercomprising: determining if the delay-lock loop is being reset when thememory device is in the standby mode; and in response to determiningthat the delay-lock loop is being reset, coupling the reference clocksignal to the delay line for a reset period before isolating thereference clock signal from the delay line.
 26. The method of claim 23,further comprising: determining if memory device is transitioning fromthe standby mode to the normal operating mode; and in response todetermining that the memory device is transitioning from the standbymode to the normal operating mode, coupling the reference clock signalto the delay line for at least a predetermined period.
 27. The method ofclaim 23 wherein the standby mode comprises a refresh mode in whichrefresh signals are generated internally in the memory device, andwherein the method further comprises: determining if memory device istransitioning from a predetermined refresh mode; and in response todetermining that the memory device is transitioning out of thepredetermined refresh mode, coupling the reference clock signal to thedelay line for at least a predetermined period.
 28. The method of claim23 wherein the predetermined refresh mode comprises a self-refresh mode.29. The method of claim 23 wherein the delay-lock loop further comprisea phase detector having first and second input terminals receiving thereference clock signal and an output signal from the delay line, thephase detector being operable to generate an output signal that controlsthe delay of the delay line, the method further comprising selectivelyenabling and disabling the phase detector when the memory device isoperating in the standby mode.
 30. The method of claim 29 wherein thememory device is operable in an on die termination mode, and wherein themethod further comprises disabling the phase detector in the on dietermination mode while coupling the reference clock signal to the delayline when the memory device is operating in the standby mode.
 31. Themethod of claim 23 wherein the memory device comprises a mode registerthat is programmable responsive to a load mode command, and wherein themethod further comprises: detecting the load mode command, and when inthe standby mode, coupling the reference clock signal to the delay lineused in the delay-lock loop responsive to detecting the load modecommand.